The present invention relates to color television image display systems and particularly to dynamic convergence circuits for the color television image display system which permits convergence of all scanned points in the four corner areas of a raster independently from one another.
An illustrative example of a color television display system is a projection type color image display system wherein three primary color images are superimposed on a viewing screen projected from respective kinescopes of different colors, or a multiple beam color kinescope of the type having a shadow mask or aperture grille. In the projection type color display system, the horizontal and vertical deflection waveforms for each kinescope tend to have different amplitudes and linearities from those of another kinescope and the horizontal and vertical axes of each raster tend to tilt at different angles resulting in a trapezoidal, or raster distortion differing from another. The three kinescopes are located with respect to a viewing screen so that the images on the kinescope screens are projected upwardly to allow the image formed on the viewing screen to find paths to the viewers' eyes. Therefore, the raster distortion tends to increase linearly as a function of the angular deflection of the beam from the lower to upper edges of the viewing screen. This is the misconvergence of projection type color display systems and conventional methods for eliminating such misconvergence involve modulating the horizontal sawtooth wave signal with the vertical sawtooth wave signal and applying the modulated signal to a convergence coil. However, due to errors arising from tolerances in manufacturing deflection coils and electron guns of each kinescope and tolerances in mounting each kinescope with respect to another, the adjustment involved is of a tedious, time-consuming affair to achieve convergence at all scanned points in the four corner areas of the raster.
In the multiple beam color kinescope having an aperture mask, on the other hand, the multiple beams are not converged at scanned points in areas off the center of the kinescope screen and the amount of misconvergence increases nonlinearly as a function of the angular deflection of the beams from the center of the kinescope screen. The usual practice is to generate current waveforms of a substantially parabolic shape from horizontal vertical deflection circuits and to employ them in conjunction with convergence control windings to dynamically converge the beams at all points of the scanned area of the kinescope screen, as disclosed in U.S. Pat. Nos. 3,393,343 and 3,500,113.
However, satisfactory results of convergences are difficult to achieve with respect to scanned points adjacent to the corner areas of the kinescope screen. This stems from the fact that the magnetic field intensity required to converge the beams at each corner area of the screen is not equal to the sum of the magnetic fields generated by the corresponding horizontal and vertical correction currents. U.S. Pat. No. 3,803,444 granted Apr. 9, 1974 to Gerritsen et al discloses a convergence circuit comprising substantially non-anisotropic astigmatic deflection coils to generate a quadripolar field which is superimposed on the normal deflection field to achieve convergence at all points in the corners of the screen. However, at least one additional specially designed deflection coil having coil halves is required to produce the intended result. This means that the use of deflection coils of the conventional design would not result in an acceptable level of convergence.